The aim of this study is to determine whether iodo-estrogens labelled with I125 and/or I131 can be used in detection and eventually in therapy of mammary tumors. It is known that about 30-40 percent of breast tumors are estrogen dependent, and there appears to be a high degree of correlation between this estrogen dependency and the ability of the tumor tissue, both primary and metastatic, to bind estrogens when studied in vitro. Should the strongly radioactive iodo- estrogens, the synthesis of which is outlined in this proposal, retain their ability to bind to neoplastic mammary tumors, a new and useful method would be available to differentiate without surgical intervention between the estrogen dependent and independent tumors. Perhaps, most important, however would be that a new approach of internal direct irradiation of breast neoplasia would become possible.